1. Field of the Invention
This invention relates to coatings, primarily for use in connection with the exterior surfaces of optical windows used in connection with transmission of infrared light.
2. Brief Description of the Prior Art
Most infrared (IR) sensors operating in the 8 to 12 .mu.m wavelength region use optical windows as a protective measure, most such windows being made from n-doped polycrystalline germanium. Since the optical refractive index of germanium is high, the reflectance from each surface is high and the net transmittance through the germanium is relatively low. The refractive index of germanium is 4.0, resulting in 36% reflectance per surface. The transmittance of uncoated germanium is only 47% through a 0.040 inch thick piece. In order to improve the IR transmittance of the window, a suitable antireflection coating is generally provided whereby the window reflectance is reduced to an average value of 0.5% or less, thereby raising the transmittance to an average value of 95% or more over the 8 to 12 .mu.m wavelength interval.
A further reason for reducing the reflectance per surface on an IR window is less apparent but equally important. Whenever a lens or window surface is nearly perpendicular to the optical axis, the cryogenically cooled IR detector can see a warm image of itself reflected from this lens or surface. A fraction of 1% reflectance from a surface about 200.degree. C. warmer than the detector can cause a spurious "noise" which blinds the system to the scene beyond the reflecting surface. This effect is called "cold spike" or "narcissism" in IR systems and can also occur on lenses, regardless of prudent lens design technologies which minimize the problem by changing air spacings and/or the radii of curvature of the lenses. The window is in front of the detector in collimated space, so there are fewer opportunities to minimize "cold spike". The worst case occurs when the system optics are mounted on gimbals and look through a multi-pane window system. There are numerous gimbal orientations where the instantaneous optical axis is sufficiently perpendicular to one of the panes to cause such reflection. It is therefore important that each IR window surface have reflectance less than 1%.
The design of multilayer coatings for optical windows and lenses begins with the work of John Strong, "On a Method of Decreasing the Reflection From Nonmetallic Substances", Journal of the Optical Society of America, Vol. 26, page 73 (1936), Cox et al. "Infrared Filters of Anti-reflected Si, Ge, InAs and InSb", Journal of the Optical Society of America, Vol 51, page 1406 (1961) and progressing to Lubezky et al., "Efficient and Durable AR Coatings for Ge in the 8-11.5 .mu.m Band Using Synthesized Refractive Indices of Evaporation of Homogeneous Mixtures", Applied Optics, Vol. 22, page 1828 (June, 1983). The recurring theme, backed by rigorous theory, is that the outermost layer of a truly efficient antireflection coating must have a very low refractive index. This layer is typically 2.0 .mu.m thick when used for 8 to 12 .mu.m applications. It must be physically hard, insoluble in water and survive in harsh environments such as salt spray and cycles of high to low temperature in a humid atmosphere. The summary article of Aguilera et al., "Antireflection Coatings for Germanium IR Optics: A Comparison of Numerical Design Methods", Applied Optics, Vol. 27, page 2832 (July, 1988) shows the extreme complexity required to even calculate an efficient design when a low index of refraction is not available. These complex designs are very sensitive to manufacturing errors and have very low yield.
The low index of refraction material of choice for most manufacturers has been thorium fluoride, ThF.sub.4. In bulk form, fluorides tend to be insoluble and have a longwave cutoff in the 6 to 12 .mu.m range. Fluorides tend to form evaporated films with high tensile stress, causing numerous problems such as cracking, peeling, and failure in harsh environmental tests. ThF.sub.4 has low solubility, forms stable films 2.0 .mu.m thick, has low absorptance in the 8 to 12 .mu.m band and adheres to most other films. Mild radioactivity is its only flaw. Accordingly, use of ThF.sub.4 requires an AEC license and strict adherence to the safety codes of Federal and State agencies.
Thorium fluoride has been used for at least 50 years in various optical applications, perhaps starting with Dimmick Patent No. 2,422,954. Dimmick uses thorium fluoride along with zinc sulfide films for visible optical purposes. The material is useful in ultraviolet (UV) filters also. The principal use of thorium fluoride has been for 8 to 12 .mu.m IR antireflection coatings. Thorium fluoride has been employed in exterior window surface coatings which must have average reflectance as low as 0.5% per surface. Other methods, such as the use of diamond-like carbon as shown by Holland Patent No. 4,382,100, produce extremely durable films but have 3% average reflectance per surface. In many systems, this reflectance value is unacceptably high on the exterior window surface. In addition, the carbon films absorb 3% in this wavelength band. The net loss caused by reflectance and absorptance is 5%. This is unacceptably high in some systems.
Continued use of thorium fluoride now presents a problem due to its radioactivity and new requirements for disposal of radioactive materials. It is therefore necessary to find other materials to substitute for thorium fluoride which can provide the same properties.
The interior surface of the window is reasonably well protected. There are more coating design options for this surface since less durability is required. Materials other than thorium fluoride, such as barium fluoride or strontium fluoride, will suffice under these conditions, and can be incorporated into coating designs producing 0.5% average reflectance.
Magnesium fluoride, MgF.sub.2, has been widely used as a durable single layer coating for visible glass optics since 1938, after the work of Cartwright and Turner Patent No. 2,207,656. It is also the low index material in nearly every visible glass multilayer coating, as shown by Thelen Patent No. 3,185,020 or Sulzbach Patent No. 3,565,509. As used for visible applications, the film is 0.1 .mu.m thick, and has very high tensile stress.
When the conventionally evaporated MgF.sub.2 film is made thicker than 0.3 .mu.m, the high tensile stress has caused cracking. This material has seldom been used for IR applications beyond the 1 to 3 .mu.m range. It is used as a bulk material in the 2 to 6 .mu.m range. A 1 cm thick piece transmits 88% at 6 .mu.m but less than 1% at 9 .mu.m. The transmittance (T) of 1 cm pieces of other materials referenced are listed in the following table:
______________________________________ Material %T @ 8 um %T @ 12 um ______________________________________ MgF.sub.2 15 0 CaF.sub.2 85 0 SrF.sub.2 92 6 PbF.sub.2 90 30 BaF.sub.2 93 40 ______________________________________
Materials which transmit well at 12 .mu.m and have a low index of refraction are very soft, very soluble or both. NaCl (common table salt) or KBr are such materials. Based upon transmittance data and physical thickness limits caused by tensile stress, MgF.sub.2 is not an obvious choice as a coating material for use in the 8 to 12 .mu.m interval.
Gibson, "Ion Beam Processing of Optical Thin Films", Physics of Thin Films, Vol. 13, page 139, Academic Press (1987), cites unpublished work indicating that 1.6 .mu.m thick crack-free films of MgF.sub.2 were possible using Ion Aided Deposition (IAD). Gibson further states that the limit of transparency of MgF.sub.2 extends only to 6.5 .mu.m, probably referring to published bulk data. Accordingly, the prior art has taught away from the use of MgF.sub.2 as an external coating for an optical window operating in the 8 to 12 .mu.m range.